Problem: The drama club sold bags of candy and cookies to raise money for the spring show. Bags of candy cost $$8.00$, and bags of cookies cost $$2.50$, and sales equaled $$31.00$ in total. There were $4$ more bags of cookies than candy sold. Find the number of bags of candy and cookies sold by the drama club.
Let $x$ equal the number of bags of candy and $y$ equal the number of bags of cookies. The system of equations is then: ${8x+2.5y = 31}$ ${y = x+4}$ Since we already have solved for $y$ in terms of $x$ , we can use substitution to solve for $x$ and $y$ Substitute ${x+4}$ for $y$ in the first equation. ${8x + 2.5}{(x+4)}{= 31}$ Simplify and solve for $x$ $ 8x+2.5x + 10 = 31 $ $ 10.5x+10 = 31 $ $ 10.5x = 21 $ $ x = \dfrac{21}{10.5} $ ${x = 2}$ Now that you know ${x = 2}$ , plug it back into $ {y = x+4}$ to find $y$ ${y = }{(2)}{ + 4}$ ${y = 6}$ You can also plug ${x = 2}$ into $ {8x+2.5y = 31}$ and get the same answer for $y$ ${8}{(2)}{ + 2.5y = 31}$ ${y = 6}$ $2$ bags of candy and $6$ bags of cookies were sold.